Sheet metal panels formed by a stamping process, plastic or non-metallic panels formed by injection molding and other similar parts often show dents, low spots and other geometric distortions from irregularities in the dies or molds used to manufacture the part or from handling damage. These may manifest themselves as indents, outdents, creases, buckles, spring back, high spots, low spots, dish shapes, tears and a myriad of other similar defects which must be detected in the inspection process. Such inspection is generally done before painting or plating the parts, that is before a significant amount of additional money is spent on the part or its assembly. However, subsequent handling and the assembly (e.g. welding) or paint processes themselves also can impart damage, requiring reinspection.
Such panels are often inspected visually in an environment of florescent linear lighting which assists the inspector to determine the quality of the panel by observing distortions in the reflected image of the lights. However, the florescent lights are of relatively low contrast and in any case, such inspection is slow and subjective. It is therefore very desirable to have a method for rapidly, automatically and objectively evaluating such defects (both for audit and 100% inspection purposes). Such rapid, quantitative analysis of defects is important for control of processes to achieve uniform flow of quality product in just-in-time production systems and to achieve uniform acceptance standards between vendors and customers.
There are numerous optical methods to measure the contour of the part that could in theory discriminate such flaws, triangulation or light sectioning for example. Another technique along with imaging a grill or grid of lines through the panel was described in the article by Lippincott and Stark, Aug. 15, 1982, Applied Optics. A similar electro-optical sensor actually constructed for inspection of body panel flaws was described in U.S. Pat. No. 4,394,683 by two of the inventors and their colleagues, which patent is incorporated herein by reference. This uses deviation of grid lines imaged through the panel and a varient is described in FIG. 13 of this patent. This works reasonably well but signal to noise levels are often low, especially on poorly highlighted panels. In addition, relatively low angles of incidence to panels are necessitated which makes operation difficult in many cases.